It is widely known to use blowing agents to foam, or produce a cellular structure within, thermoplastic materials. The goal in using a blowing agent is to make the thermoplastic article of lower density and lighter in weight. Such characteristics as lower weight enable the thermoplastic articles to be used more extensively in applications such as aircraft and space vehicles.
The method currently used to foam thermoplastics is to add to a thermoplastic a blowing agent and/or other materials which volatilize and generate gas which permeates the thermoplastic and forms an internal cellular structure within the thermoplastic article. Many of the blowing agents used are chemical blowing agents which react with the thermoplastic through thermal or chemical decomposition in order to generate the required gas.
High temperature thermoplastic engineering polymers are useful in modern applications because of their great temperature resistance, their ability to be reinforced with fibrous or nonfibrous materials, and their ability to be processed at reasonable temperatures.
In foaming high temperature engineering polymers conventional blowing agents volatilize at too low a temperature to be highly effective. Upon decomposition within a high temperature thermoplastic, the gas generated merely escapes the mixture and will not form the desired cellular structure because the thermoplastic will not have reached its melting temperature and cannot flow to form cells. Even when blowing agents which have decomposition temperatures about the melting point of high-temperature thermoplastics are used, other problems, such as lack of oxygen in a closed mold or insufficient gas production, are still present.
In in-situ fabrication of high-temperature thermoplastic advanced composites, high internal pressure is needed to consolidate the sandwich skin(s). Due to the high operating and molding pressures needed for production and due to the difference in thermal coefficients of expansion of the materials, cracks and voids may form in the structure if insufficient pressure is present. Failure to produce sufficient in-situ pressure to consolidate the article will also result in an inconsistent foaming action.
The present invention is directed to overcoming one or more of the above problems.